In a radio communication system using radio technology, a communication terminal realizes a network connection by exchanging various kinds of data by establishing synchronization with the synchronizing signal sent out from a radio base station.
FIG. 1 is a diagram showing one mode of a typical radio communication system.
The radio communication system shown in FIG. 1 includes terminal 1001, base station 1002 and network 1003.
Terminal 1001 is a communication terminal having a radio communication function. For example, a mobile phone and a mobile terminal can be mentioned.
Base station 1002 is a radio base station that connects terminal 1001 with network 1003.
Network 1003 is a communication network, for example, may be a WiMAX (Worldwide Interoperability for Microwave Access) network.
Here, data to be transmitted and received between terminal 1001 and base station 1002 have to be transmitted and received in synchronization with the aforementioned synchronizing signal. Accordingly, terminal 1001 needs to detect the time (temporal position) of transmission of the synchronizing signal transmitted from base station 1002.
In order for terminal 1001 to detect the synchronizing signal transmitted from base station 1002, a search operation of searching the synchronizing signal is performed in a predetermined period of time (e.g., time for two frames) (see JP2000-175238A, for example).
FIG. 2 is a diagram showing a format of data frames transmitted and received between terminal 1001 and base station 1002 when network 1003 shown in FIG. 1 is a WiMAX network.
As shown in FIG. 2, the data frame transmitted and received between terminal 1001 and base station 1002 is configured of a preamble indicating the lead of the data frame, DL or downlink data, UL or uplink data and TTG/RTG or Gap time for absorbing propagation delay between terminal 1001 and base station 1002. Here the preamble is used, as the synchronizing signal for establishing synchronization between terminal 1001 and base station 1002. This data frame is transmitted and received between base station 1002 and terminal 1001 in a predetermined cycle (in a cycle of 5 ms in a WiMAX system).
FIG. 3 is a diagram showing one example of the timing of a typical synchronizing signal searching operation of terminal 1001 in the radio communication system shown in FIG. 1. In this case, description will be made taking an example where the PR (preamble) at the lead of the data frame transmitted from base station 1002 is used as the synchronizing signal.
First, a synchronizing signal searching process is performed at frequency f1 for the period of two frame intervals (time ta). At this time, a search process is performed at terminal 1001 for time ta with the radio Power set to ON and the receiver set to ON.
If no synchronizing signal is detected at frequency f1, the frequency is switched to frequency f2 and a further synchronizing signal searching process is performed for time ta with the radio Power set to ON and the receiver set to ON.
When the synchronizing signal is detected at frequency f2, the synchronizing signal search process will take the total search time shown in FIG. 3.
In this way, since the position of the synchronizing signal has not been able to be known at terminal 1001, it is necessary to perform a searching process for the predetermined time. Accordingly, problems have occurred in which power that is consumed in the search process is wasted and in which the search process takes time.